A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device because its bandgap is sufficiently wide. Among other things, gallium nitride-based compound semiconductors which contain Ga as a Group III element (which will be referred to herein as “GaN-based semiconductors” and which are represented by the formula AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1)) have been researched and developed particularly extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlxGayInzN (where 0≦x, y, z≦1 and x+y+z=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. As for the classification of the “c-plane”, a c-plane which is terminated with a Group III element, such as Ga, is called either a “+c-plane” or a “(0001) plane”, and a c-plane which is terminated with a Group V element, such as nitrogen, is called either a “−c-plane” or a “(000-1) plane”. It should be noted that the “c-axis” and the “c-plane” are sometimes referred to as “C-axis” and “C-plane”.
In fabricating a semiconductor device using GaN-based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is used as a substrate on which GaN semiconductor crystals will be grown. In a c-plane, however, Ga atoms and nitrogen atoms are not present in the same atomic plane, thus producing electrical polarization there. That is why the c-plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated in the InGaN quantum well of the active layer in the c-axis direction. Once such a piezoelectric field has been generated in the active layer, some positional deviation occurs in the distributions of electrons and holes in the active layer. Consequently, the internal quantum yield decreases due to the quantum confinement Stark effect of carriers, thus increasing the threshold current in a semiconductor laser diode and increasing the power dissipation and decreasing the luminous efficacy in an LED. Meanwhile, as the density of injected carriers increases, the piezoelectric field is screened, thus varying the emission wavelength, too.
Thus, to overcome these problems, it has been proposed that a substrate of which the principal surface is a non-polar plane such as a (10-10) plane that is perpendicular to the [10-10] direction and that is called an “m-plane” (m-plane GaN-based substrate) be used. As used herein, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). As shown in FIG. 2, the m-plane is parallel to the c-axis (i.e., the primitive vector c) and intersects with the c-plane at right angles. On the m-plane, Ga atoms and nitrogen atoms are on the same atomic plane. For that reason, no spontaneous polarization will be produced perpendicularly to the m-plane. That is why if a semiconductor multilayer structure is formed perpendicularly to the m-plane, no piezoelectric field will be generated in the active layer, thus overcoming the problems described above. The “m-plane” is a generic term that collectively refers to a family of planes including (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes.
Also, as used herein, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X=c or m) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be referred to herein as a “growing plane”. A layer of semiconductor crystals that have been formed as a result of the X-plane growth will be referred to herein as an “X-plane semiconductor layer”.
The light-emitting device which is fabricated using a nitride semiconductor can be classified into two device types, the first device type where the p-electrode and the n-electrode are arranged on the same crystal growth surface side (horizontal type) and the second device type where the p-electrode and the n-electrode are vertically arranged so as to sandwich a nitride semiconductor layer (vertical type), irrespective of whether c-plane or m-plane is used for the principal surface. The horizontal type can be further classified into the horizontal face-up type where light goes out from the p-type nitride semiconductor layer side and the horizontal face-down type where light goes out from the n-type nitride semiconductor layer side.
In the case of the horizontal type, the p-electrode and the n-electrode are largely horizontally separated from each other. Therefore, the current concentrates in an area where the space between the p-electrode and the n-electrode is smallest, so that uniform current injection into the active layer is difficult. Due to this electric field concentration, the breakdown voltage of the light-emitting device disadvantageously decreases.
On the other hand, the vertical type is also referred to as “attached type”. In the vertical type nitride-based semiconductor light-emitting device, light goes out from the n-type nitride semiconductor layer side while the p-type nitride semiconductor layer side is attached onto a conductive substrate. In the case of the vertical type, the n-electrode blocks light, and therefore, the n-electrode should be as small as possible. In the n-type nitride semiconductor layer, electrons are locally injected at a position which is in contact with the n-electrode. Therefore, if electrons do not sufficiently diffuse in the n-type nitride semiconductor layer, uniform current injection in the active layer will be difficult. To realize uniform current injection in the active layer, a configuration where a transparent electrode is used as the n-electrode has already been put to practical use. However, this configuration causes another problem that light is absorbed by the transparent electrode.
As described above, either of the horizontal type structure and the vertical type structure has a disadvantage that, without sufficient horizontal diffusion of electrons in the n-type nitride semiconductor layer, the current-voltage characteristic would deteriorate (i.e., the operating voltage would increase).
Usually, horizontal diffusion of electrons is realized by increasing the donor impurity concentration of the n-type nitride semiconductor layer. However, in the n-type nitride semiconductor layer formed on the c-plane according to the prior art, if the impurity concentration is increased to a high value which is, for example, higher than 1×1019 cm−3, the crystallinity disadvantageously deteriorates (see, for example, Patent Document 1). Therefore, in many of the nitride-based semiconductor light-emitting devices which have already been put to practical use, the impurity concentration of the n-type nitride semiconductor layer is not more than 5×1018 cm−3.
To realize a low forward voltage (Vf) and a high emission power, Patent Document 2 proposes a configuration where the n-type nitride semiconductor layer is formed by multiple n-type GaN layers of different impurity concentrations. Since Patent Document 2 describes using a c-plane sapphire substrate, the principal surface of the n-type nitride semiconductor layer is probably a c-plane surface.
FIG. 3 is a cross-sectional view showing a prior art nitride-based semiconductor light-emitting device disclosed in Patent Document 2. In the nitride-based semiconductor light-emitting device shown in FIG. 3, a buffer layer 102, an n-type GaN underlayer 103, an n-type contact layer 104, an active layer 105, a p-type cladding layer 106, and a p-type contact layer 107 are provided on a substrate 101. An n-side pad electrode 109 is provided so as to be in contact with the n-type contact layer 104. A transparent electrode 108 is provided so as to be in contact with the p-type contact layer 107. A p-side pad electrode 110 is provided so as to be in contact with the transparent electrode 108. Between the n-type contact layer 104 and the active layer 105, a multi-film nitride semiconductor layer 111 is interposed so as to be in contact with the active layer 105. The multi-film nitride semiconductor layer 111 is a multilayer film formed by two or more repetitions of a multilayer structure consisting of the first nitride semiconductor layer 111a and the second nitride semiconductor layer 111b. The first nitride semiconductor layer 111a is a layer which contains an n-type impurity. The second nitride semiconductor layer 111b is a layer which contains an n-type impurity at a concentration lower than the first nitride semiconductor layer 111a, or an undoped layer. Patent Document 2 discloses that the emission power can be improved while a low forward voltage (Vf) is maintained. Although details of the reasons why the emission power improves are not clear from Patent Document 2, an estimated reason is that the efficiency of carriers injected into the active layer improves.
To provide an n-type nitride semiconductor layer of a high doping concentration and high crystallinity, Patent Document 3 proposes a multilayer structure that includes a plurality of n-type GaN layers which have doping concentrations exceeding 1×1019 cm−3 and a plurality of undoped GaN layers which have thicknesses of 30 nm or more. Patent Document 3 fails to describe the plane orientation of the principal surface of the n-type nitride semiconductor layer.
FIG. 4 is a cross-sectional view showing a prior art nitride-based semiconductor light-emitting device disclosed in Patent Document 3. In the nitride-based semiconductor light-emitting device shown in FIG. 4, a buffer layer 202, an n-type nitride semiconductor layer 203, an active layer 204, and a p-type nitride semiconductor layer 205 are provided on a substrate 201. An n-electrode 208 is provided so as to be in contact with the n-type nitride semiconductor layer 203. A p-electrode 206 is provided so as to be in contact with the p-type nitride semiconductor layer 205. A p-side bonding pad 207 is provided on the p-electrode 206. The n-type nitride semiconductor layer 203 includes a multilayer structure which is formed by a plurality of n-type GaN layers 203a and a plurality of un-GaN layers 203b. In the configuration of Patent Document 3 where a plurality of undoped GaN layers which have thicknesses of 30 nm or more are provided, the crystallinity of the n-type semiconductor layer which has been once degraded due to a high impurity concentration can be recovered.
Patent Document 4 discloses a “heavily doped layer” but fails to describe an n-type GaN substrate. Patent Document 4 also fails to describe the crystallinity of a current diffusing layer that is formed on an n-type GaN substrate so as to be in contact with the n-type GaN substrate or the crystallinity of a semiconductor multilayer structure formed on the current diffusing layer.